s3:smb2_server: generate a header blob for the sendfile path

s3:smb2_server: generate a header blob for the sendfile path

We need to pass the NBT header, SMB2 header and SMB2 Read header
as header blob to SMB_VFS_SENDFILE(). This allows the usage
of MSG_SEND or other tricks to avoid multiple TCP packets
on the wire.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: David Disseldorp <ddiss@samba.org>